As indicated in U.S. Pat. No. 4,078,286, in a typical process for electrophotographic duplication, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member, and the latent image is subsequently rendered visible by the application of electroscopic particles, which are commonly referred to as toner. The visible toner image is then in a loose, powdered form and it can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support which may be the photosensitive member itself or another support such as a sheet of plain paper. A principle aspect of the present invention relates to the fusing of the toner image upon a support.
In order to fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
The use of thermal energy for fixing toner images onto a support member is well known. Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means: a roll pair maintained in pressure contact; a flat or curved plate member in pressure contact with a roll; a belt member in pressure contact with a roll; and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art, and they can be adjusted to suit particular machines or process conditions.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to affect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an abhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont de Nemours under the trademark TEFLON. The silicone rubbers which can be used as the surface of the fuser member can be classified into three groups according to the vulcanization method and temperature, i.e. room temperature vulcanization silicone rubber hereinafter referred to as RTV silicone rubber, liquid injection moldable or extrudable silicone rubber, and high temperature vulcanization type silicone rubber, referred to as HTV rubber. All these silicone rubbers or elastomers are well known in the art and are commerically available.
One of the more common roll fuser materials comprises either fuser rolls or pressure rolls made from a condensation cured polyorganosiloxane which is filled with finely divided iron oxide to provide thermal conductivity and stability throughout the silicone rubber layer and also impregnated with a small amount, up to about 10% for example of a low viscosity, about 100 centistokes, silicone oil. Fuser and pressure rolls made from such a composition are capable of performing satisfactorily but they have their useful life limited inasmuch as the fusing operation occurs at a temperature in the region of 400.degree. F. and oxidative crosslinking of the methyl groups on the polyorganosiloxane rubber will result in a higher crosslinked polysiloxane producing lower toughness, lower elongation, a higher modulus material and a lower fatigue life. The mechanism of the oxidative degradation of the polyorganosiloxane is not fully understood, however, it is believed that at elevated temperatures (of the order of 400.degree. F.) such as are employed in the fusing of toner images on a copy sheet, the oxygen attacks the methyl groups of the siloxane elastomer oxidizing them thereby permitting oxidative crosslink hardening of the material. The attack by oxygen is believed to create free radicals which by further reaction eventually results in a silicon-oxygen-silicon crosslink thereby hardening the elastomer. In particular, the toughness, (determined from a plot of stress versus strain indicating the amount of energy it takes to fracture the elastomer from tensile stress and elongation) the area under the stress strain curve may be reduced dramatically leading to fuser roll failure. For example, a typical material impregnated with about 10% by volume silicone oil as a release material has an initial toughness of about 300 in-lbs/in.sup.3. However, after fusing between 8,000 to 32,000 copies at a temperature of the order of 400.degree. F., the toughness was reduced to 140 in-lbs/in.sup.3 which is unsatisfactory and the silicone oil level has been reduced to about 2% which resulted in the loss of release properties. With the oxidative crosslinking, the hardening of the elastomer takes place resulting in cracking, pitting, and eventually fracturing of the rubber layer at the core resulting in catastrophic failure of the fuser roll. While initially the cracks or pits may appear only at the surface of the roll, other cracks and flaws present throughout the rubber layer can propagate sufficiently to cause cohesive failure whereby portions of the rubber come off the roll causing failure of the fuser and surrounding fuser elements. Further, since the amount of silicone release fluid has been depleted, the surface energetics are higher and therefore release of the toner material from the fuser roll becomes more difficult. In addition, with increased hardening of the fuser roll the elasticity in the fuser roll decreases and the ability to release the toner to the paper becomes degraded resulting in a more mottled copy quality in the resulting copies. Furthermore, with increased surface energetics the probability of the fuser roll picking up paper debris and other contaminants which will attract toner is increased.